Guarantee representation of None in NPO

rust-lang · Oct 14, 2023 · 9703cb2 · 9703cb2
1 parent 2a7c2df
commit 9703cb2
Showing 1 changed file with 15 additions and 9 deletions.
diff --git a/library/core/src/option.rs b/library/core/src/option.rs
@@ -119,15 119,21 @@
 //! # Representation
 //!
 //! Rust guarantees to optimize the following types `T` such that
-//! [`Option<T>`] has the same size and alignment as `T`:
-//!
-//! * [`Box<U>`]
-//! * `&U`
-//! * `&mut U`
-//! * `fn`, `extern "C" fn`[^extern_fn]
-//! * [`num::NonZero*`]
-//! * [`ptr::NonNull<U>`]
-//! * `#[repr(transparent)]` struct around one of the types in this list.
 //! [`Option<T>`] has the same size and alignment as `T`. In some
 //! of these cases, Rust further guarantees that
 //! `transmute::<_, Option<T>>([0u8; size_of::<T>()])` is sound and
 //! produces `Option::<T>::None`. These cases are identified by the
 //! second column:
 //!
 //! | `T` | `transmute::<_, Option<T>>([0u8; size_of::<T>()])` sound? |
 //! |---------------------------------------------------------------------|----------------------------------------------------------------------|
 //! | [`Box<U>`] | when `U: Sized` |
 //! | `&U` | when `U: Sized` |
 //! | `&mut U` | when `U: Sized` |
 //! | `fn`, `extern "C" fn`[^extern_fn] | always |
 //! | [`num::NonZero*`] | always |
 //! | [`ptr::NonNull<U>`] | when `U: Sized` |
 //! | `#[repr(transparent)]` struct around one of the types in this list. | when it holds for the inner type |
 //!
 //! [^extern_fn]: this remains true for any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)
 //!